1. Field of the Invention
The present invention relates to a redundant switchover system for a time division multiple access (TDMA) device.
2. Description of the Prior Art
In a TDMA satellite communication system, messages are relayed from earth station to earth station via a common communication satellite by using the same up link and down link frequency, respectively. In this case, a TDMA frame is divided into a plurality of time slots allocated to the earth stations. Therefore, each earth station transmits its own burst in an allocated time slot for the satellite, and each earth station receives bursts from the satellite.
The transmit timing of the burst of each station is determined according to reference bursts transmitted by so-called reference stations. That is, each earth station continuously monitors the reference bursts and its own burst and controls its own burst transmit timing to maintain its own burst in the allocated slot at the satellite. Such control for burst transmit timing is called transmit burst synchronization, and is most important for a TDMA satellite communication system.
Some earth stations have a redundant configuration. That is, each earth station has a double-structure comprising an active TDMA terminal unit and a stand-by TDMA terminal unit. When a fault is detected in the active terminal unit, the active terminal unit is switched to the stand-by unit, thereby continuing communication via the satellite.
In more detail, the above-mentioned active and stand-by TDMA terminal units are combined by a switching/distributing portion and a common radio frequency (RF) portion connected to an antenna. Therefore, if a fault is generated in the switching/distributing portion or the common RF portion, such a fault cannot be saved or corrected, even when the active unit is switched to the stand-by unit. On the other hand, if a fault is generated in the active unit itself, such a fault can be saved or avoided by switching the active unit to the stand-by unit.
In the detection of such a fault, one approach is for the earth station to monitor the presence or absence of its own burst signals, i.e., the transmit burst synchronization state, by detecting a unique word of its own burst included in the received bursts. If no burst is detected in the time slot allocated to its own burst, the active unit declares that the transmit burst synchronization is lost. This loss of transmit burst synchronization is mainly due to three phenomena, as follows:
(I) The transmit portion of the active TDMA terminal unit contains a fault which can be detected by other alarm monitor means.
(II) The transmit portion of the common RF portion contains a fault.
(III) The transmit portion of the active TDMA terminal unit contains a fault which cannot be detected by other alarm monitor means. For example, the transmit burst includes an erroneous unique word or burst identification code, a transmit burst timing deviated from the allowable zone, inadequate modulation, or a cable disconnection between the active unit and the switching portion.
For phenomenon I, a loss of the transmit burst synchronization can be saved or corrected by the switchover from the active unit to the stand-by unit caused by the alarm detection means. For phenomenon II, a loss of the synchronization cannot be saved or corrected by switching the active unit to the stand-by unit. Also, for phenomenon III, a loss of the synchronization cannot be saved or corrected, since both the active and stand-by units carry out a detecting operation for the loss of the synchronization upon the received bursts, simultaneously, so that the stand-by unit as well as the active unit declares that the transmit burst synchronization is lost. Therefore, for phenomena II and III, communication is suspended to carry out a burst transmit timing acquiring operation, i.e., an initial aquisition.